The present invention generally relates to radio frequency communication systems and, more particularly, to a site diversity technique for providing high link availability on a satellite or other radio frequency (RF) link that is challenged by thin power margins and high probability of deep rain fade.
Site diversity refers generally to a set of techniques used to provide high link availability on a satellite or other RF link that is challenged by thin power margins and high probability of deep rain fades, such as a wideband bandwidth efficient modulation (BEM) link. It is not always possible to locate a BEM receive station in a low precipitation area of the world. In some cases, it may not be possible to build a link with ample power margin to overcome all high or moderate probability rain fades. Rather than over-sizing the link to carry large rain-fade power margins, which could be cost-prohibitive, site diversity may be desirable to lower the required power margin and increase link availability. Using site diversity, the communication system receives the transmitted signal into two or more geographically separated receive locations, generally within the same downlink beam. These receive locations are separated far enough in distance to provide decorrelation of atmospheric phenomena so that; for example, if it is raining hard on one receiver (producing a deep rain fade), it is probably not raining as hard (thus lower rain fade) over another receiver. Thus, by suitably combining information from both or all received signals it is generally possible to reconstruct the original information from the transmitted signal with a greater degree of accuracy than would be possible using only one signal. Thus, link availability, e.g., the percentage of time that a link can provide a signal of acceptable quality for accurate reconstruction of the original information, can be increased in comparison to a link that uses only one receiver. But previous methods of combining multiple received signals are usually impractical or very costly for a very high data rate BEM link.
For example, prior art techniques for site diversity include one that involves coherently combining the received RF or intermediate frequency (IF) waveforms (referred to as “coherent waveform combining”) and another that involves demodulating the received data streams and making “either-or” decisions down stream about which packets to accept from which stream (referred to as “digital switching”).
To perform the first technique, coherent waveform combining—assuming for the sake of example, two receive stations—some component of the communication system must perfectly align and then combine both RF signals at one common location with negligible distortion. Such perfect alignment is not feasible for ultra-wideband communications, especially with BEM formats that are highly distortion-sensitive.
The other technique, digital “either-or” switching, requires very fast decision-making capabilities, which can be either not feasible or cost prohibitive for the high frequencies and data rates involved. Moreover, the process of decoding the signals involves at one stage making a “soft decision” about the data symbol received and then refining the soft decision into a “hard decision” at a later stage of the decoding process. Digital “either-or” switching requires very fast decision-making capabilities, and does not provide optimal performance, since decoding must be performed on the individual data streams, leaving only the information from the “best” stream to be kept, and any additional possible performance benefit of the other stream is wasted.
As can be seen, there is a need for optimally combining two or more received signals to obtain a single received data stream without the difficulties of RF coherent combining and without the performance loss of digital switching methods. There is also a need for an efficient, low cost, low complexity, and high performance method of combining two BEM downlinks separated by significant distance that achieves optimal rain-fade resistance. Moreover, there is a need for BEM signal combining in which not only is the unit cost low, but the total system requirements, and therefore the total system cost, is also very low.